My signal in the uk is terrible. My note 9 had no problems at all. And all my samsung galaxys before have never had this issue. Im so disappointed and there seems to be no fix fir the issue
I think I have solved the problem. I have a German Galaxy S21 Ultra and I did the following:
1. Factory Reset, made sure all latest updates are installed
2. I wiped the System Cache in the Android Recovery menu
3. Setup my phone as new, not from a backup or copying old data (not sure if this had any effect)
The phone is now super fast, no lags or stuttering and no mobile data connection problems. Try and let me know if it works for you. Best of luck!